"Back in the day", cast iron pans were manufactured in a much more labor-intensive way. Each sand mold (minimum of 2 per item) was hand-rammed around a form, which was a wood (later aluminum) "positive" of the pan to be produced. The forms were slightly larger than the finished pan to allow for the shrinkage of the iron as it cooled. Molten iron was poured by hand into the forms, which is as much art as science to do properly. After the iron hardened the frames of the molds were removed, and the "raw" pan was ready for machining. First the "gates" on the edge of the pan were removed by nipping/grinding. The pan was then placed in an apparatus similar to a brake drum lathe and turned. A counter-turning grinding burr (shaped as a truncated cone) was run across the cooking surface of the pan. The quality of the result was due to the fineness of the sand used in the mold, the age/quality of the grinding burr, and the skill of the machinist. The bumps didn't get "smoothed out with use". To my knowdledge, none of the major hollow-ware manufacturers sand-blasted any if their products.
In the late 50's, early 60's, domestic manufacturers had to compete with imports if cheap overseas manufacture. Labor overhead made the old manufacturing methods economically unviable. The surviving manufacturers, BSR and Lodge, retooled for automated casting. This led to the thicker, unmilled pans that are with us today.
The spiel about "the rough surface is for pre-seasoning" us pure marketing BS. I bought Lodge dutch ovens in the 70's that had the rough surface and NO PRE-SEASONING. They were, however, shipped with a thin coat of paraffin wax to prevent rusting. Lodge came up with the "pre-seasoning" story years after they started selling un-milled pans (and people complained about how tough it was removing the wax coating).
Older, smoothly milled, properly seasoned pans are WAY slicker than you can get any rough surface iron. My daily egg-fryer is an unmarked Lodge #5 from the 40's. It was a $5 crusted-up wreck of a thrift store find. Degreased in a lye bath, further cleaned by electrolysis, and re-seasoned with 6 baked coats of flax seed oil. With a wipe of oil, over medium heat, the cooked eggs slide around in the pan, nearly as slick as Teflon.
Furthermore, cast iron does NOT do a good job distributing heat, but DOES do a good job retaining heat. Copper, and even cast aluminum, are better heat conductors/distributors. A thinner cast iron pan works just as well as thick one. The only thing that thicker, rough finish cast iron does better than the smooth pans is sear meat.